Default parameters in a spawned function

Questions / Help
1

In this module if I call Test.func directly, the default parameter is picked up, but if I run Test.start, it is ignored:

defmodule Test do

  def start do
    _pid = spawn(__MODULE__, :func, [[], []])
  end

  def func(list, index \\ 0 ) do
    IO.puts "index: #{index}, list: #{inspect list}"
  end
end

iex(1)> Test.func([:a])
index: 0, list: [:a]
:ok
iex(2)> Test.start
#PID<0.94.0>
index: , list: []
iex(3)>

Is there any way around this?

2

When you spawn your function, you are giving 2 arguments, so there is no need to apply the default ones, it should work after changing the spawn: spawn(__MODULE__, :func, [[]])

Also, are those backslashes in your code as well, or are they a copypaste accident?

3

The extra backslashes were there because I couldn’t get the back ticks to work from my iPad. Someone else must have added them.

But I need that default value when the func is called. I need to use the index as an index into the list, and elem(index, list) complains rather vociferously when index doesn’t have a value.

(And the backticks seem to be working now…)

4

I had inserted the backticks for you, it was easier for me to read the code then :wink:

And to make my example from the first post more clear:

defmodule Test do
  def start do
    _pid = spawn(__MODULE__, :func, [[]])
  end

  def func(list, index \\ 0 ) do
    IO.puts "index: #{index}, list: #{inspect list}"
  end
end

This should do what you want. In your example, you are overriding the default, by providing 2 arguments in the list of arguments. If you wan’t the default to kick in, you are not allowed to provide more than one argument.

Your call in spawn, were like func([], []).

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